Fluorine-containing elastomers, particularly perfluoro elastomers mainly comprising a tetrafluoroethylene (TFE) unit are widely used as a sealing material to be used under hard environment because of excellent chemical resistance, solvent resistance and heat resistance thereof.
However requirements for characteristics thereof required with advance of technology are becoming strict. In the fields of space aeronautics, semi-conductor production apparatuses and chemical plants, a sealing property under high temperature environment of 300° C. or more is demanded.
To cope with such requirements, proposals have been made to enhance heat resistance by improving a crosslinking system. As such a crosslinking system, there are known crosslinking methods, for example, a method of triazine crosslinking by forming a triazine ring with an organotin compound by using a fluorine-containing elastomer having a nitrile group introduced as a crosslinking point (for example, JP-A-58-152041), a method of oxazol crosslinking by forming an oxazol ring with bisaminophenol similarly by using a fluorine-containing elastomer having a nitrile group introduced as a crosslinking point (for example, JP-A-59-109546), a method of imidazole crosslinking by forming an imidazole ring with a bisdiaminophenyl compound (for example, JP-A-59-109546) and a method of thiazole crosslinking by forming a thiazole ring with bisaminothiophenol (for example, JP-A-8-104789).
In a series of PCT patent applications (WO97/19982, WO98/23653, WO98/23654 and WO98/23655) of Du Pont, U.S.A., it is proposed that an end of a fluorine-containing elastomer having nitrile group is converted to a carbonyl-containing end group for the purpose to increase a crosslinking rate in peroxide crosslinking system in addition to the above-mentioned triazine crosslinking system and oxazole crosslinking system.
However a crosslinked rubber obtained by a crosslinking system described in JP-A-58-152041, JP-A-59-109546 and JP-A-8-104789 is insufficient in a mechanical strength and compression set at high temperature because a crosslinkable functional group is present only in a branched chain derived from a cure site monomer.
An essential object of the inventions disclosed in the above-mentioned applications of Du Pont is to decrease the number of sulfonic acid end groups which may cause a crosslinking failure. To achieve such an object, the end groups are converted to carbonyl-containing groups. Namely, the crosslinking rate is enhanced by decreasing the number of sulfonic acid groups but not by using carbonyl-containing groups as crosslinking points. This can be seen from the description that it is preferable to decarboxylate carbonyl-containing end groups by heating to decrease a viscosity of the elastomer because the carbonyl-containing end groups ionized or to be ionized increase the viscosity of elastomer. In those publications, carboxyl group, carboxylic acid salt and carboxyamide group are raised as a carbonyl-containing end group. However since a metal salt is used for coagulation of the obtained emulsified polymer, carbonyl-containing end groups of the coagulated and isolated elastomer to be subjected to crosslinking have been converted to metal salts of carboxylic acid or carboxyamide group. It is assumed that those salts cause an increase in a viscosity of the elastomer. The decarboxylation treatment of the carbonyl-containing end groups indicates that the end groups are not used for crosslinking.
Further also in the crosslinking system of the invention of Du Pont, in which a fluorine- and nitrile-containing elastomer having a carbonyl-containing group as an end group is used, a mechanical strength and compression set at high temperature of the obtained crosslinked product are not improved.
An object of the present invention is to provide a novel crosslinking system of a fluorine-containing elastomer giving a crosslinked product having improved mechanical strength and compression set at high temperature.